This invention relates to a control for the heater of a heated oxygen sensor in an internal combustion engine closed loop air/fuel ratio control, and especially for such a control utilizing a zirconia oxygen sensor or its equivalent. Such air/fuel ratio controls are becomming ever more common in emission control systems wherein they help maintain a substantially stoichiometric air/fuel ratio to the engine so that the exhaust gases may be successfully treated by a three-way catalytic converter.
Such closed loop air/fuel ratio control systems generally utilize such a sensor exposed to the exhaust gases from the engine and sensitive to at least one component thereof. A typical such sensor is the zirconia oxygen sensor, which operates as an electrochemical cell and, when warm, generates a typical output voltage of 800 to 1,000 millivolts in the absence of oxygen that would be characteristic of exhaust gases from an air/fuel ratio richer than stoichiometric and generates a typical output voltage of 0 to 200 millivolts in the presence of oxygen characteristic of exhaust gases obtained from an air/fuel mixture leaner than stoichiometric.
However, the internal impedance and generated voltage of such sensors varies greatly with temperature, such that the sensor must be heated far above normal atmospheric environmental temperatures to generate a useable output voltage. In most cases, the heat of the exhaust gases from the engine is sufficient to heat the sensor to the required operating temperature and maintain such temperature; however, when an engine is started from a cold condition there is a period of time before the sensor becomes sufficiently heated to operate correctly in the control system. In addition, different engines vary considerably in exhaust temperature; and some engines may have an exhaust temperature so cool that the sensor temperature falls below its desired operating temperature during some conditions of engine operation, such as prolonged idle. Most closed loop air/fuel ratio control systems provide for such periods of sensor unreliability due to insufficient sensor temperature by substituting open loop backup controls at the proper times. However, in most cases, such open loop controls do not provide the efficiency of emission reduction provided by the closed loop system; and it therefore may be desirable to minimize such periods of open loop control if it is necessary to increase the overall efficiency of the emission control.
One method of minimizing the periods of sensor unreliability due to low sensor temperature is the use of an electric heater to heat the oxygen sensor directly and thus control its operating temperature. A good example of an oxygen sensor of the type described above which includes a heater element is shown in the U.S. Pat. No. 4,178,222 issued to Michael P. Murphy et al on Dec. 11, 1979. The incorporation of the heater directly into the sensor package provides a compact, self-contained unit and good heat conduction between the heater element and sensor. However, the variation in exhaust temperature over time from a single engine and the efficiency of heat conduction in such a sensor design make it undesirable, in most cases, for the heater to be operated at all times. It is most desirable to operate the heater only when sensor temperature falls below a predetermined temperature or when the engine is being operated in a condition which makes it likely that the sensor temperature would fall below the predetermined temperature.
In addition, a zirconia oxygen sensor is characterized by a charge storage effect when supplied with current from an external source. This effect may be described in terms of an internal capacitance in parallel with its output terminals. When a heater element is combined in a single package with such a sensor, it is difficult to avoid a leakage current path from the heater to the sensor terminals in either the sensor itself or the connecting means. This leakage current path may have a very high resistance; but, nevertheless, the heater current may be sufficient to appreciably charge the sensor capacitance and thus affect the sensor output voltage. This further supports the desirability, at least in some systems, of not operating the oxygen sensor heater at all times.